1. Technical Field of the Invention
The present invention relates to a method of making a battery plate for lead acid storage batteries, especially a method for making a battery plate using a grid comprising a lead-calcium-tin alloy.
2. Prior Art
Hitherto, a lead-antimony (Pb--Sb) alloy has been mainly used as a grid or battery plate for lead acid storage batteries, but the batteries using the alloy as grid suffer from the problems that the amount of an electrolyte decreases due to overcharge of the batteries occurs owing to antimony. Therefore, a lead-calcium-tin (Pb--Ca--Sn) alloy free from the self-discharge has been increasingly used as the alloy for the grid.
Production of a battery plate using a grid of such a Pb--Ca--Sn alloy is disclosed in U.S. Pat. Nos. 3,881,952 and 4,279,977.
Generally, battery plates are produced by expanding a rolled sheet of the Pb--Ca--Sn alloy to form a grid and filling the expanded grid with an active material paste. Furthermore, the resulting expanded grid is kept at a strength desired by the plate by age hardening it.
However, a lead acid storage battery using a Pb--Ca--Sn alloy grid as the battery plate becomes a lower contact between the grid and the positive active material than a lead acid storage battery using a grid of the Pb--Sb alloy as the battery plate, and thus the former battery has a problem of deterioration in battery characteristics such as deep charge-discharge cycle life at high temperatures.
One cause for such a problem is considered that the Pb--Ca--Sn alloy is oxidized and corroded during the use of the battery and, as a result, it grows or deforms to make a much lower contact between the grid and the positive active material.
Furthermore, since the stress strain is concentrated on the nodes of the grid when subjected to the expanding process, the stress strain concentration accelerates the corrosion of the nodes and these nodes are selectively more rapidly corroded as compared with the other portions.
A method of improving the contact between the grid and the positive active material is proposed by U.S. Pat. No. 4,906,540 in which a thin film of a Pb--Sb alloy is applied to the surface of a grid of a Pb--Ca--Sn alloy and the film is integrated to the grid.
However, conventional methods of making the expanded grid of the Pb--Ca--Sn alloy cannot satisfy both the workability in expanding of the alloy in a sheet form and the strength of the resulting grids. That is, the Pb--Ca--Sn alloy sheet can easily be expanded when the sheet is soft, but some alloy compositions do not provide a strength enough to stand the fabrication of the battery even after subjected to aging.
On the contrary, when the Pb--Ca--Sn alloy sheet is hard, there is the problem that the expanding of the sheet is difficult.
Owing to this problem, when the mechanical strength of the expanded grid is low, the grid grows or deforms to a large extent during the operation of the battery, resulting in the lowering of contact between the grid and the active material and reduction of the cycle life of the battery.
When the strength of the expanded grid is high, stress strain is concentrated on the nodes of the grid when expanded and hence the corrosion of the nodes rapidly proceeds.
The effect of a thin film of a Pb--Sb alloy applied to the surface of the grid of a Pb--Ca--Sn alloy is reduced in half if the corrosion resistance and strength of the grid as base of the Pb--Ca--Sn alloy are insufficient.